A unique approach to treating HIV and HTLV-I infections has been initiated using a system of gene therapy which provides "intracellular vaccination". The herpes thymidine kinase (HSV-tk) gene has been placed under the regulatory control of the HIV-LTR, which includes the TAR regulatory sequence in an MSV packaging-defective viral vector containing a selectable marker for neomycin resistance. Stable 3T3-derived cell lines containing this construct and packaging-defective virus have been produced. Studies will determine whether the HSV-tk enzyme will be synthesized in response to the HIV Tat trans-acting protein. Cells containing the HSVtk enzyme will selectively metabolize specific drug analogues (such as acyclovir) to a toxic metabolite, which will specifically kill those dividing cells. Thus, any Tat-producing cell infected by HIV can be selectively destroyed, while leaving a normal, uninfected stem cell population to replenish the lost T-cells. Studies will be conducted to establish cell lines stably transformed with the HIV-LTR-HSVtk construct. The HIV-LTR-HSV-tk T-cell lines will be infected with live HIV virus. Toxicity response to acyclovir will be established for live virus infection. Gene therapy utilizing human cells will be tested in SCID mice. The HIVLTR-tk construct in the defective retroviral vector will infect and integrate into human bone marrow cells. Resistance to HIV infection with acyclovir administration will be tested in vivo. Similar studies for HTLV-I will be conducted.